1. Field of the Invention
The present invention relates to a velocity control device applied to a velocity control system, for example, a numeral control machine.
2. Description of the Related Art
FIG. 6 is a block diagram of a conventional velocity control device which controls a velocity of an object mechanically connected to a motor, by controlling a velocity of a servo motor. To a velocity control device 1, a velocity command value V is supplied by a precedent command device (not shown in the figure). A subtracter 100 subtracts a motor velocity vm from the velocity command value V and calculates a velocity deviation V-vm. Here, the velocity deviation is amplified by an operation of PI (proportional integration) amplification described below, and becomes a torque command value .tau.c. The velocity deviation V-vm is amplified by a factor of a proportional gain Kp in an amplifier 101 and produces a proportional component .tau.p. Furthermore, the velocity deviation is amplified by a factor of an integral gain Ki in an amplifier 102 and becomes d .tau.i/dt, is integrated in an integrator 103, and produces an integral component .tau.i. The .tau.p and the .tau.i are added together in an adder 104 to become a torque command value .tau.C.
A power amplifying section 105 is comprises an electric power amplifier (not shown in the figure) and a servo motor (also not shown), and is a section to amplify a torque command value .tau.c to a motor output torque .tau., and the amplification factor thereof is expressed by a torque conversion constant Ct. An object system 107 comprises a motor and a controlled object (not shown in the figure) mechanically connected to the motor. A disturbance torque .tau.d is a disturbance torque acting on the object system from outside, and is added to the motor output torque .tau. in an adder 106 shown equivalently, and finally, a torque acted on the object system becomes .tau.+.tau.d. A position detector (not shown in the figure) or a velocity detector (not shown in the figure) is connected to the motor and, on the basis of the detected information thereof, a motor velocity vm showing a velocity of a controlled object can be obtained.
Here, provided that the controlled object is a rigid body and the motor and the controlled object are rigidly connected, by using a total inertia moment J of the motor and the controlled object, a velocity response of a conventional velocity control device 1 shown in FIG. 6 can be expressed by the following expression.
Expression 1 EQU {(V-vm)Kp+.intg.Ki(V-vm)dt+.tau.i(0)}Ct+.tau.d=J(dvm/dt)
when .tau.i(0)=vm(0)=0 are initial conditions, when Laplace transformation is carried out, the following expression can be obtained from Expression 1 (wherein S shows an operator of Laplace transformation indicating a derivative action, and 2 shows square).
Expression 2 ##EQU1## Here, a damping factor .zeta. of a generally used normal quadratic form and a natural frequency .omega.n of the system are defined as follows (wherein { } (1/2) shows the one-half power of { }).
Expression 3 EQU .zeta.=(Kp/2){Ct/(Ki.multidot.J)} (1/2) EQU .omega.n={KiCt/J} (1/2)
If Expression 2 is expressed, being divided into a command response characteristic when letting .tau.d=0, and a disturbance suppression characteristic when letting V=0, it becomes the following expressions.
Expression 4 EQU vm(S)=[{2.zeta..omega.nS+.omega.n 2}/{S 2+2.zeta..omega.nS+.omega.n 2}]V(S)
Expression 5 EQU vm(S)=[(S/J) {S 2+2.zeta..omega.nS+.omega.n 2}].tau.d(S)
That is, as for a conventional velocity control device shown in FIG. 6, Expression 4 expresses the command response characteristic and Expression 5 expresses the disturbance suppression characteristic.
From the above description, it is clear that in a velocity control device, a command response characteristic and a disturbance suppression characteristic can be variable by operating a proportional gain Kp and an integral gain Ki, and it is further clear from Expression 5 that setting a large integral gain Ki is effective for improving disturbance suppression ability. However, if an integral gain Ki is simply set large, a damping factor .zeta. is simultaneously lowered and, therefore, damping characteristics of the command response characteristic shown by Expression 4 are worsened. Moreover, since a reduced damping factor arises under certain frequencies, swelling phenomena of a specified frequency at a specified velocity have an arisen under the influence of rotational errors or the like of a position detector having a frequency proportional to the velocity.